Osteoarthritis (OA) is one of the most common musculoskeletal disorders in older individuals. Profound aging- associated changes in chondrocytes and cartilage extracellular matrix represent a major factor in OA pathogenesis. Although referred to as a degenerative joint disease, OA is an active cartilage remodeling process. Cells in OA cartilage are activated and not only produce matrix degrading enzymes, but also new extracellular matrix. The remodeling process ultimately fails because of an imbalance between matrix synthesis and degradation, because the newly formed cartilage is non-hyaline repair tissue and matrix is often abnormally calcified. Our preliminary studies suggest that adult human articular cartilage contains chondroprogenitor cells. The proposed experiments are based on the following hypotheses: 1) Mesenchymal progenitor cells are present in adult cartilage, and they can be isolated, expanded and induced to differentiate to multiple mature mesenchymal phenotypes. 2) The frequency, localization, activation and response patterns of mesenchymal progenitor cells are altered in aging and osteoarthritis. 3) The genetic basis of mesenchymal cell differentiation from the progenitor to the discrete subsets of mature chondrocytes can be established with DNA microarrays to define markers and regulators of this process. The following specific aims will test these hypotheses: 1) Determine the frequency and localization of progenitor cells in normal human articular cartilage using immunohistochemistry, cell isolation, flow cytometry and functional assessment of pluripotential differentiation. With these studies, we will develop markers for identification of progenitor cells and methods for isolation and expansion. 2) Analyze the presence of progenitor cells in aging and osteoarthritic human cartilage. Determine whether there is an aging- associated reduction in the number of progenitor cells, whether the cells have an impaired responsiveness to growth factors and whether progenitor cells are associated with the clusters of proliferating chondrocytes and areas of calcified cartilage. 3) With the DNA microarrays, define the genetic basis for the chondrocyte subsets in the various zones of normal human articular cartilage. Establish the relationship between bone marrow derived and cartilage derived progenitors and differentiated chondrocytes generated from them. The discovery of progenitor cells in adult cartilage opens new perspectives to harness these cells in cartilage repair and raises the possibility that aging- associated changes in their frequency or function determine joint aging and that they contribute to the aberrant cartilage remodeling process in arthritis.